Imaging systems are employed in a variety of applications, such as medical devices, satellite as well as microscope and telescope apparatuses. More recently, imaging systems have been utilized in a variety of additional applications such as digital cameras, computer scanners and digital camcorders. A majority of these applications utilize charge-coupled-devices (CCDs) as the underlying image sensors. However, CCD-based image sensors are limited or impractical for use in many consumer applications.
For example, CCDs typically are manufactured using four to six inch wafer fabrication lines, whereas many fabrication processes employ twelve to fourteen inch lines. CCDs thus cannot be fabricated employing conventional Complimentary-Symmetry Metal-Oxide Semiconductor (CMOS) fabrication processes. The mismatch in processing technology for CCDs also precludes the integration of additional on-chip functionality beyond light sensing and charge to voltage conversion for CCDs. As a result, CCDs usually employ independent support chips to perform, for example, CCD control, A/D conversion, and signal processing, such as implemented with CMOS integrated circuits (ICs).
The operation of CCD imagers also requires multiple high supply voltages (e.g., 5V-12V) resulting in higher power consumption than CMOS imagers. As a result, the costs for the CCD image sensor and the system employing the sensor remain high. Additionally, since the current to charge the CCDs is high, CCDs are not well suited for portable devices.
CMOS imagers have offered improvements in functionality, power and cost in many applications (e.g., digital video, digital cameras). A CMOS type image sensor includes a photodiode or phototransistor employed as a light detecting element. These sensors use active pixels, hence their alternate name Active Pixel Sensors (APS). Each pixel has an amplifier that converts the collected charge packet to a voltage. The output of the light detecting element is an analog signal whose magnitude is approximately proportional to the amount of light received by the elements. The magnitude of the analog signal can be measured for each photo diode representing a pixel and stored to provide an entire stored image. CMOS imagers utilize less power, have lower fabrications costs and offer high system integration compared to imagers made with CCD processes. Additionally, CMOS imagers have the advantage that they can be manufactured using similar processes employed to those commonly used to manufacture logic transistors, such that the necessary CMOS imager support functions can be fabricated on the same chip.
The potential to achieve wide dynamic range imaging of CMOS image sensors have also attracted attention in the field of electronic imaging that was previously dominated by CCDs. Dynamic range is the ratio between the brightest and darkest recordable parts of an image or scene. Several techniques have been utilized in an effort to improve the dynamic range of conventional CMOS imagers that implement voltage domain sampling. Some of these methods include logarithmic response CMOS imagers, multiple frame capture techniques, and floating-point pixel-level ADC imagers. Logarithmic response CMOS imagers incorporate logarithm compression at the photodiode level to achieve wide dynamic range. The logarithmic response technique suffers from the problem of fixed pattern noise due to the device mismatches, and poor sensitivity and local contrast. Multiple frame capture techniques implement a lateral overflow gate to increase pixel dynamic range. This technique suffers from mismatch in the lateral overflow transistor gate-drain overlap capacitance. Also it requires capturing multiple frames and complex reconstruction processing. Furthermore, its logarithmic compression curve strongly reduces image contrast. Floating-point pixel-level ADC imagers require large memory to store the data and require a complex reconstruction process.
Recently, some researchers have started to explore time domain sampling techniques in order to overcome the inherent limitations of conventional CMOS imagers operating in the voltage domain. Voltage-to-frequency conversion photosensors provide a high dynamic range. However, the readout process of the photosensor array generally takes a very long time. Thus, this technique is not applicable for many implementations. In another approach, an arbitrated address event representation digital image sensor technique utilizes row and column arbiters to send out the pixels according to the firing order. This technique requires a high-resolution timer and a large frame buffer. Pixel-parallel analog-to-digital (A/D) conversion CMOS imagers implement a free-running photocurrent-controlled oscillator to give a first-order Σ-Δ sequence. This technique requires a constant reference voltage and the imaging procedure requires a full second of time. A time domain quantization sensing (TDQS) technique uses the idea of digitizing a sensing pixel analog value by quantizing it in the time domain. A scene is sampled multiple times in the TDQS system and a large memory is needed to store the data. Also, since a pixel is read off-chip multiple times, the power consumption of the system can be substantial.